Radio-frequency linear accelerators (RF linacs) can generate high average current electron beams used in many important applications such as high average power free-electron laser sources, intense x-ray sources, positron sources, high frequency (harmonic) RF sources, and terahertz (THz) sources. These sources in turn have major commercial, defense, and homeland security applications such as sterilization, sensing of contraband and special nuclear materials, directed energy applications, and materials processing.
An RF linac consists of an electron source followed by a series of RF accelerating cavities that raise the energy of the injected electrons to the level required by the particular application. The electron source, often referred to in the art as an “electron injector” can consist of just a cathode (either thermionic, field emission, or photoemission) in the wall of the first accelerating cavity, or may be a more complicated structure that does its own initial electron acceleration and bunching, for example, with DC or RF fields, prior to injecting the electrons into the main linac.
The properties of the electron injector play a major role in determining the properties of the electron beam produced by the linac, including the electron beam's energy spread, transverse emittance, and temporal structure (i.e., microbunch length). The average current available from any given RF linac is primarily limited by the cw average currents available from the electron source used, but it can also be affected by other factors such as beam loading of the RF cavities, wall heating (in the case of normally conducting cavities), and the effect of electrons that return to the cathode of the injector as a result of being injected into the linac at an incorrect RF phase for capture and acceleration in the linac.
A high average current linac is one in which the total electron charge accelerated in a single period of the rf drive is high, a large fraction of the rf periods are filled with electron charge while the rf drive is on (ideally, all of the rf periods are filled), and the rf drive is continuous in time, rather than present only in short duration rf pulses. By these means, the cw average current of the linac will be high and therefore suitable for high average current applications.
One particularly important application of a high average current RF linac is as an electron source for a high average power infrared free electron laser (FEL). See, e.g., Phillip Sprangle, Joseph Peñano, Bahman Hafizi, Daniel Gordon, Steven Gold, Antonio Ting, and Chad Mitchell, Phys. Rev. ST Accel. Beams, vol. 14, pp. 020702-1-020702-15.
A typical FEL comprises a high average current RF linac such as an energy recovery linac (ERL), a wiggler magnet, optical components, and a beam dump for the spent electron beam. The operating parameters of the FEL impose significant requirements on the quality of the electron beam input into the RF linac from the electron injector. The electron injector must provide a high current relativistic electron beam in which the electrons are in the form of bunches that are short compared to the RF period associated with operating frequency of the RF linac. For example, for a high power FEL, every RF bucket in the ERL must be filled with charge, and so for a 700 MHz RF linac with no subharmonic section, the electron injector must generate electron bunches of order 100 psec in pulse length at a 700 MHz pulse repetition rate.
Moreover, in order to produce an average current of ˜1 A, the instantaneous current should be about an order of magnitude higher, with the charge per bunch on the order of 1 nC or higher. For such short bunches and high repetition rate, it is not practical to generate short high voltage pulses to apply to the grid of an electron gun, and direct RF modulation of the cathode-grid gap is required.
Several types of electron injectors have been used with RF linacs within the existing state of the art. These include thermionic injectors using DC high voltage electron guns, RF thermionic or field emission injectors, and laser photocathode injectors. Each of these has major limitations that do not permit high current operation at ˜1 A average current.
For example, thermionic and field emission cathodes without grids have no method to directly gate the electron emission. In the presence of an RF field, they will emit electrons over 180 degrees of RF phase, and thus do not support the short pulse format required for formation of electron micropulses having the necessary characteristics. The best current technology electron injectors for low average power FELs use laser photocathode electron guns and conventional first harmonic RF structures. However, this technology cannot be scaled to produce the required average beam current of ˜1-2 A because the low quantum efficiency of the cathodes would require very high average laser powers to create the high average current beam. See e.g., S. J. Russell, “Overview of high-brightness, high-average-current photoinjectors for FELs,” Nucl. Inst. and Methods Phys. Res. A 507, p. 304 (2003) In fact, these injectors have not yet demonstrated even ˜100 mA of average beam current.
There are two additional problems with extending laser photocathode technology to generate high average current beams. First, the required lasers do not exist, and second, the required laser power, if it were available, would destroy the cathode due to excessive thermal loading. Thus, such electron injectors are not suitable for use with a 1 MW FEL.
Gridded thermionic electron guns also have been used as electron sources for RF linacs, and are capable of direct modulation at frequencies of order 1 GHz. These guns use barium dispenser cathodes and pyrolytic graphite grids, and are well known to the state of the art, since they are used as part of commercial RF amplifier tubes known as inductive output tubes (IOTs). In the IOT, the gun operates with a negative bias of ˜30-40 kV between the cathode and the anode, which is at ground potential, as well as with a small relative bias of order −100 V on the grid with respect to the cathode. The grid bias serves to prevent electron emission from the cathode until an RF signal of sufficient amplitude is induced between the cathode and grid. In the presence of such an RF signal, emission only takes place when the RF phase is such that the RF field overcomes the negative bias on the grid, producing a train of short electron bunches synchronized with the RF signal. In an IOT gun, the beam would then be accelerated up to an energy of tens of keV by the DC negative bias between the cathode and the grounded anode, and the beam extracted through the anode would be used to generate RF power by deceleration in an output cavity. This gridded thermionic electron gun thus produces inherently low energy electrons and relatively long micropulses that are not suitable for use in an FEL.
Thus, there is a need for a new electron injector capable of generating high quality relativistic electron bunches for further acceleration in a high average current RF linac such as an ERL. This injector would replace the other means of generating the initial electron bunches required , such as laser-photocathodes injectors or DC or RF thermionic injectors.